Add .editorconfig file

This will enforce a couple of code conventions
(indentation: 2 spaces, \n for EOL, utf-8 charset)
similar to .vscode/settings.json, but with the
EditorConfig format (https://editorconfig.org/)
which is supported by most major IDEs.

.editorconfig

@@ -0,0 +1,11 @@
+# EditorConfig is awesome: http://EditorConfig.org
+
+# top-most EditorConfig file
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
+charset = utf-8
+end_of_line = lf